Pigments that contribute light-scattering properties to coatings are generally known as white, or hiding, pigments. They act by scattering all wavelengths of light, owing to their relatively high refractive index, so that they are perceived as white to the human eye. The most widely used white pigment is titanium dioxide (TiO2), a polymorphous substance that exists in three modifications or crystal structures, rutile, anatase or brookite. Only the anatase and rutile modifications are of any note, technically or commercially.
The high demand for titanium dioxide based pigments is driven by a combination of a high refractive index and a reasonable manufacturing cost. Additionally, titanium dioxide based pigment does not suffer from the same environmental considerations as earlier white pigments such as Lead carbonate, which had a high toxicity and were readily released into the environment when placed in contact with water.
The anatase phase of titanium dioxide has a lower refractive index and is generally less durable than the rutile form, which makes it less desirable as a coating pigment. However, as will be seen below, both the lower refractive index and lower durability are highly desirable in some applications.
Although the most important use for titanium dioxide is as a pigment, the material is in fact colourless. To reveal its special properties, the titanium dioxide must first be processed to a certain particle size. For example, for pigment applications the particle size would be one half the wavelength of visible light or about 0.3 microns.
Aside from its excellent properties as a pigment, titanium dioxide has dielectric properties, high ultraviolet absorption and high stability which allows it to be used in speciality applications, such as Electro-ceramics, glass and as an insulator.
Titanium dioxide pigments are used in man-made fibres, such as polyester, viscose and rayon to name a few. As man made fibres have an undesirable glossy and translucent appearance, the pigment is incorporated into the fibre during the spinning process either for brightening the fibre or reducing the fibre's lustre. For this application the anatase phase is advantageous since as it has a more neutral white tone than the rutile modification and is also less abrasive. This latter property is very important as the process for spinning fibres is very delicate and would be adversely affected by the addition of the rutile form of titanium dioxide to the fibres. Anatase, on the other hand, is a photo catalyst that is activated by ultraviolet radiation resulting in the rapid degradation of the man made fiber when exposed to sunlight.
Titanium dioxide is also used for adding opacity and brightness to plastics. The opaqueness and high brightness help mask the poor natural colour of many plastics. Additionally, some grades of titanium dioxide absorb ultraviolet light which can accelerate the ageing of plastics.
Additionally, titanium dioxide is added as a filler to the pulp in paper manufacturing processes to enhance brightness and opaqueness. This allows, for example, for the production of highly opaque lightweight papers. For this application titanium dioxide in its anatase phase can be used.
In order to manufacture titanium dioxide, a source of titanium is required. Although titanium ranks ninth in abundance among elements found in the crust of the earth, it is never found in the pure state. Rather, it occurs as an oxide in the minerals ilmenite (FeTiO3), rutile (TiO2) or sphene (CaO—TiO2—SiO2).
The production of titanium dioxide pigments is a two step process. The first step is to purify the ore, and is basically a refinement step. This may be achieved by either the sulphate process, which uses sulphuric acid as a liberating agent or the chloride process, which uses chlorine as the liberating agent.
In the sulphate process, the titanium containing ore is dissolved in sulphuric acid, yielding a solution of titanium, iron, and other metal sulphates. Through a series of steps including chemical reduction, purification, precipitation, washing, and calcination, pigment size TiO2 is produced.
Alternatively, the chloride process includes high-temperature, anhydrous vapour phase reactions. Titanium ore is reacted with chlorine gas under reducing conditions to obtain titanium tetrachloride (TiCl4) and metallic chloride impurities, which are subsequently removed. Highly purified TiCl4 is then oxidized at high temperature to produce intermediate TiO2. The oxidation step in the chloride process permits control of particle size distribution and crystal type, making it possible to produce high quality pigment grade TiO2.
The chloride process is inherently cleaner than the sulphate process and requires a smaller investment on behalf of the manufacturer in terms of waste treatment facilities. Additionally, titanium dioxide produced using the chlorine process is generally of higher purity, more durable and has a particle size distribution which is narrower, the latter improving brightness, gloss and opacity.
As stated above, the chloride process includes high-temperature anhydrous vapour phase reactions where liquid titanium tetrachloride is vaporised and superheated after which it is reacted with hot oxygen to produce titanium dioxide. The superheating and subsequent reaction phase can be carried out either by a refractory process, where the reactants are heated by refractory heat exchangers and combined. Alternatively, carbon monoxide can be purified and then mixed with the titanium tetrachloride and oxidizing agent and then the mixture subject to a controlled combustion. Finally, the titanium tetrachloride can be vaporised in a hot plasma flame along with the oxidizing agent. This final method has proven to be the most efficient.
A number of technical approaches are available for generating the plasma. For example, the plasma may be generated by passing the working gas between a pair of electrodes whereby an arc discharge ionizes the gas as it passes between. A drawback of this approach is that the electrode is bound to contaminate the working gas, either by trace chemical reaction between the electrode and the working gas, or by degradation of the electrodes. This drawback is particularly acute when the working gas is an inert, reducing or oxidizing gas.
U.S. Pat. No. 5,935,293, entitled “Fast Quench Reactor Method” issued to Detering et al. on Aug. 10, 1999 described a method for producing ultra-fine solid particles in an electrode-generated plasma reactor. The reactor is configured so as to cause a metal halide reactant stream introduced in the reactor to expand after reaching a predetermined reaction temperature thereby causing rapid cooling thereof. The expansion results from the stream passing through a quench zone where the stream reaches supersonic velocity. The quench zone is intended to prevent back reaction and promote completion of the reaction.
A major drawback of the Detering method, in addition to the above-mentioned contamination problem, is that it does not lend itself for a reactant dilution sufficiently high for the generation of nanopowders, and to avoid powder agglomeration. Indeed, electrode-generated plasma are known to be relatively high-energy and to yield non-uniform temperature in the reactor. Those two conditions prevent the use of important dilution of reactant and render difficult control on the particle size distribution. It is to be noted that the Detering method, when used in the synthesis of TiO2, does not promote the production of its anatase phase.
In other known methods, the working gas may be passed through a high frequency electrostatic field. According to other known methods, the working gas may be passed through a high frequency induction coil whereby the electromagnetic field ionizes the gas as it passes within the coil. It is to be noted that induction plasma torches are characterized by a volume discharge larger than direct current plasma source, and a longer residence time. Indeed, for comparable power rating, an induction plasma torch would operate with more than 100 standard liters per minute of plasma gas, compared with 20-30 standard liters per minute of plasma gas with electrode-generated plasma reactor.
The synthesis of pigment grade titanium dioxide through the oxidation of titanium tetrachloride in a plasma flame formed by passing a working gas through a high frequency induction coil is well known in the art and has been used industrially for some time for the commercial production of such powders for the paint industry.
Traditionally, the product obtained in this case is composed of relatively large opaque particles with a particle size in the range of 0.2 to 2.0 micrometers or more. Such powders are used as a base material for the production of a wide range of paints and surface modification coatings.
There has always been an interest in obtaining finer powders in the nanometer range for a wide variety of other applications including ultraviolet protection and the sunscreen industry as well as for advanced catalyst development. However, the development of a process to produce large quantities of titanium dioxide nanopowders has proved difficult to attain. The main obstacle has been the method to achieve such an important reduction in the size of distribution of the powder and control its chemistry and surface properties.